A turbomachine comprises statoric and rotoric bladerows, exchanging angular momentum with the fluid. A fluid with angular momentum is also called a swirling fluid. The swirl is said positive if it has the same sense of the rotating speed and negative in the opposite case.
In a turbine the statoric bladerows generate a positive angular momentum in the fluid at expenses of a pressure drop, while the rotoric bladerows extract this angular momentum from the fluid and convert it into torque on the shaft.
On the contrary, in a compressor the rotoric blades provide a positive angular momentum into the fluid at expenses of torque on the shaft, while the statoric bladerows convert this angular momentum into an increase of fluid pressure.
This mechanism is repeated for each stage, i.e. for each pair of rotoric and statoric bladerows.
In case of a compressor, the residual angular momentum after the statoric bladerows can be positive or negative or, of course, it can vanish. As a result, the downstream stage is said respectively unloaded or overloaded, as compared to a reference case where the flow has no swirl at the inlet.
As a matter of fact, a positive angular momentum at the inlet of a stage reduces the work required for providing a given amount of positive angular momentum at the exit. This means that the stage absorbs a lower power for the same mass flow rate and therefore it is said unloaded.
For the opposite reason, a negative angular momentum at the inlet of a stage increases the absorbed power for the same mass flow. In such conditions the stage is said overloaded.
Generally, as compared to the absence of inlet swirl, the polytropic head developed by a compressor stage, for a given mass flow, is a bigger quantity if the angular momentum at inlet is negative (overloaded stage) and smaller if it is positive (unloaded stage).
Due to the typical negative slope of the head-flow curve, a centrifugal compressor stage with positive swirl will deliver the same head at a lower flow than an equal stage without inlet swirl. For the opposite reason, the flow will increase for a stage with negative swirl at inlet.
On this principle the adjustable inlet guide vanes (IGV) are based: IGV control the swirl at the inlet of a stage, and in this way they increase or decrease the flow delivered for a given head. In this sense, overall IGV are a device for controlling the flow of a turbomachine.
In the field of “Oil & Gas”, multistage centrifugal compressors may be equipped with adjustable IGV at many locations inside the machine. They are typically installed in front of the first stage, but there are also cases where IGV are upstream of an intermediate stage.
As far as an intermediate stage is concerned, known IGV are defined by the rear portion a kind of moveable tail of the blades of the upstream return channel. Such tail can be pivoted around a fixed axis, thus working as IGV for the downstream stage.
In the prior art, this tail rotates about an axis substantially located close to its leading edge and there is a position—the reference one—where this tail substantially forms an integrated airfoil with the fixed part of the blade. In other words, in the prior art, the IGV for an intermediate stage is just obtained by splitting a conventional blade in two pieces and making adjustable one of them, the so-called tail. FIG. 1 shows a blade of an IGV device in two pieces with a moveable tail according to the prior art.
Known IGV devices do not fully meet the ideal requirements of controlling the flow with minimum losses and minimum actuation force, that is the force one should apply to overwhelm the resistance forces and rotate the IGV. The resistance forces comprises the friction forces inside the actuation mechanism and the forces due to the change of angular momentum of the flow. Indeed a change of the angular momentum of the flow reflects into a pressure distribution over the whole IGV profile and into a consequent torque to be overwhelmed with respect to the pivot of the IGV.
More in detail, the IGV devices of the prior art have at least two disadvantages. The first one is that the aerodynamic shape of the profile of the IVG is not optimized at positions different from the reference one. The second one is that the location of the above fixed axis, around which a tail of the IGV can rotate, does not minimize the actuation force to move the IGV.
As far as the above first disadvantage is concerned, it is evident that simply rotating the tail around its leading edge could produce undesired corners in both suction and pressure side of the integrated profile, wherein overall the integrated profile is defined by the fixed part and the adjustable part. Such corners in turns would generate considerable profile losses. These latter are particularly relevant when the IGV must provide negative angular momentum, i.e. in a condition wherein both mass flow rate and flow deflection are a maximum. In other words, similarly to the downstream stage, the IGV device itself is said overloaded for negative swirl and unloaded for positive swirl.
As far as the actuation force is concerned, instead, this is particularly high because the pivot is close to the leading edge and therefore the length of the lever arm is maximized for the majority of points along the IGV profile, where the flow applies its own pressure. This in turns makes the torque due to flow pressure particularly high.
Therefore there is a general need for an improved device for controlling the flow.